It is known, e.g. as disclosed in EP 0225753A (University of California), to generate and sustain a discharge in a gas using electromagnetic surface waves. Surface waves are created by a launcher which is positioned around and external of, but not extending the whole length of, the discharge tube containing the gas. In such an arrangement, it is not necessary to provide electrodes inside the discharge tube. The power to generate the electromagnetic wave is provided by a radio frequency (r.f.) power generator.
EP 0225753A discloses that if enough power is supplied, the surface wave can extend to the far end of the tube and be reflected. Accordingly, uniformity of radiation output from the tube can be enhanced by arranging the length and impedance of the tube to sustain a reflected surface wave from the end furthest from the launcher to nearly the front wall of the launcher. The reflected wave will excite and ionize the gas in the tube in addition to the excitation due to the original wave.